Systematic Sub-Micron Na/Ca Banding in Orbulina universa and bilobata

Elisa A Bonnin1, Zihua Zhu2, Howard J Spero3, Baerbel Hoenisch4, Ann D Russell3, Jennifer S Fehrenbacher5 and Alexander C Gagnon1, (1)University of Washington Seattle Campus, Seattle, WA, United States, (2)Pacific Northwest National Laboratory, Environmental Molecular Sciences Laboratory, Richland, WA, United States, (3)University of California Davis, Department of Earth and Planetary Sciences, Davis, CA, United States, (4)Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY, United States, (5)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States
Abstract:
Mg/Ca ratios in planktic foraminifera are used widely as a proxy for past sea-surface temperatures. However, over the last decade, it has become clear that these ratios are not constant throughout the shell. Instead these ratios vary systematically by several fold between day and night independent of temperature, a phenomenon that has yet to be explained mechanistically. Determining whether elements other than Mg also exhibit sub-micron banding is essential to properly interpret Me/Ca-based paleoproxies and could help constrain the mechanisms causing Me/Ca variability. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), an isotope mapping technique with a spatial resolution of roughly 200 nm, we discovered systematic Na/Ca banding in individuals of the symbiont-bearing planktic foraminifer Orbulina universa that had been cultured at constant temperature. Using stable-isotope time stamps, we show that this Na/Ca banding varies inversely with Mg/Ca, with high Na/Ca during the day and low Na/Ca at night for most individuals. Using a combination of analytical models and complementary instrumental techniques, we test whether these patterns can be explained by various ion transport processes. In addition to this Na/Ca banding pattern, there is a distinct region of both high Mg/Ca and high Na/Ca at the location of the primary organic membrane. This POM signature may be a useful way to map organic layers in foraminifera, a method we tested in bilobata, a rare morphotype of O. universa that develops a secondary sphere. Mapping Na/Ca and Mg/Ca in bilobata, we show that an additional organic layer is required during secondary sphere growth and that mineralization occurs over both spheres when this additional quasi-chamber forms. Applying ToF-SIMS and our new understanding of Na/Ca heterogeneity to bilobata is a first step towards connecting the extensive geochemical knowledge developed in O. universa to the multi-chambered species used in paleoceanography.
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